Abstract
This research examined the impact of eyewitness identification decisions on student-investigators. Undergraduates played the role of police investigators and interviewed student-witnesses who had been shown either a good or poor view of the perpetrator in a videotaped crime. Based on information obtained from the witness, student-investigators then chose a suspect from a database containing information about potential suspects and rated the probability that their suspect was the culprit. Investigators then administered a photo lineup to witnesses, and re-rated the probability that their suspect was guilty. Student-investigators were highly influenced by eyewitness identification decisions, typically overestimating the information gained from the identification decision (except under conditions that led witnesses to be very accurate), and were generally unable to differentiate between accurate and inaccurate witnesses.
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Notes
Another fundamental difference between police officers and jurors is that whereas the latter are laypersons with respect to the criminal justice system, police investigators are professionals, often with substantial prior experience working with witnesses. We return to this issue in the General Discussion, but for present purposes it is moot because the research we report tested mock investigators.
The experimenter was not blind to condition which is not ideal as there is the possibility that the note-taker could influence the participants. However, information was recorded in the same manner for all participants by responding in one or two word answers to each of the questions asked. Also, as indicated in the results section, there were no differences in any of the pre-identification measures to indicate that the experimenter affected any of the responses.
For example, looking only at target present lineups, a measure of accuracy can be obtained by calculating the number of identifications of the suspect divided by the number of identifications of the suspect plus foils, that is, given that the witness makes an identification, what is the probability that he or she identifies the culprit? Witnesses were significantly more likely to identify the culprit in the good view condition (56% of the time) than the poor view condition (20% of the time), z = 1.72, p = .043 (one-tailed). However, we are interested in accuracy when the suspect is present as well as when the suspect is not present as this is the case for concern in the real world. As witnesses were no more likely to make an accurate decision when they had a good view (56% of the time) than when they had a poor view (50% of the time) in target absent lineups, z = 0.31, p = .378, it was decided to collapse across viewing condition for all analyses.
Target presence is not included as a factor here to simplify the results as it was found that Fs < 1 for the main effect of target presence and for all interactions with target presence. In the real world, we wouldn’t know if the culprit was present in the lineup or not; thus, we have collapsed across TP and TA conditions. However, details of these analyses are available from the first author.
See Wells and Bradfield (1998) for an explanation of why a MANOVA is the appropriate analysis here.
Target presence is not included as a factor here to simplify the results as it was found that Fs < 1.3 for the main effect of target presence and nearly all interactions where target presence was included as a factor. For the Target × ID Decision interaction, F(1, 118) = 1.48, MSE = 506.95, p = .226, partial η2 = 0.01, and for the Time × View × Target × ID Decision interaction, F(1, 118) = 2.12, MSE = 192.36, p = .148, partial η2 = 0.02.
We thank an anonymous reviewer for bringing this contribution to our attention.
Statistical comparisons aren’t made between Dahl et al. and Study 2 as different materials were used in the two studies. However, in Study 2, pre-identification probabilities were 63.66% (SD = 20.01%) and 65.35% (SD = 16.26%) for the good view and poor view conditions respectively.
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Acknowledgements
This research was supported in part by grants from the Social Sciences and Humanities Research Council of Canada to Melissa A. Boyce, D. Stephen Lindsay and C. A. E. Brimacombe.
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Portions of this research were presented at the 2005 and 2006 meetings of the American Psychology-Law Society.
Appendix: Equations for Posterior Probability that the Suspect is the Culprit given Eyewitness Identification Decision
Appendix: Equations for Posterior Probability that the Suspect is the Culprit given Eyewitness Identification Decision
Note: S = Suspect, IDS = identification of suspect, IDfoil = identification of foil, noID = no identification made.
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Boyce, M.A., Lindsay, D.S. & Brimacombe, C.A.E. Investigating Investigators: Examining the Impact of Eyewitness Identification Evidence on Student-Investigators. Law Hum Behav 32, 439–453 (2008). https://doi.org/10.1007/s10979-007-9125-5
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DOI: https://doi.org/10.1007/s10979-007-9125-5